I work with 3D animation. For 3D rendering, the processor cores are more important. A dedicated workstation graphics card is good for viewport hardware rendered views (and certified ones make sure there's no display issues). A beefy GPU can be utilized for video rendering, video decoding, or multiple monitors.
One potential issue with 4K that hasn't been mentioned is what software you're using. Photoshop Elements and older versions of Photoshop/Lightroom don't support hiDPI (having the app's text scale with the OS's font scale setting). There is a hack you can try on programs, but since it is a hack, you'll have mixed results of display issues. Without hiDPI support, the text on a 4K laptop screen is quite tiny. For photography, I've found the main advantage of working on a 4K monitor is that the DOF is more perceptive (you can really see what's in sharp focus). Apart from some potential hiDPI software issues, there can be scaling issues with certain programs. I've seen noticable moire type patterns on clothing viewing images fit to screen with Windows image viewer.